Lesson 4.2: if, elif, and else Statements

The if Statement

The if statement is the most fundamental decision-making tool in Python. It lets you run a block of code only when a condition is True.

# Basic if statement syntax
if condition:
    # This code runs only if condition is True
    do_something()

Here is a concrete example:

temperature = 35

if temperature > 30:
    print("It's a hot day!")
    print("Stay hydrated!")

print("Weather check complete.")

Notice three critical things: (1) the condition temperature > 30 comes after if, (2) there is a colon at the end of the if line, and (3) the code that runs conditionally is indented.

Indentation Matters!

In Python, indentation is not optional — it is part of the syntax. The indented block after an if statement defines what code belongs to that condition. Use 4 spaces for each level of indentation (this is the Python standard).

score = 85

if score >= 70:
    print("You passed!")       # Part of the if block
    print("Congratulations!")  # Also part of the if block

print("Your score was:", score) # NOT part of the if block - always runs

# What happens when the condition is False?
score = 50

if score >= 70:
    print("You passed!")       # Skipped because 50 >= 70 is False
    print("Congratulations!")  # Skipped too

print("Your score was:", score) # This still runs - not indented

if-else: Two Paths

Often you want to do one thing if a condition is true and something different if it is false. The else clause handles the "otherwise" case.

age = 16

if age >= 18:
    print("You can vote!")
else:
    print("You are not old enough to vote yet.")
    years_left = 18 - age
    print(f"You can vote in {years_left} years.")

number = 7

if number % 2 == 0:
    print(f"{number} is even")
else:
    print(f"{number} is odd")

if-elif-else: Multiple Conditions

When you have more than two possible paths, use elif (short for "else if") to check additional conditions. Python checks them in order from top to bottom and runs the first one that is True.

score = 82

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
elif score >= 60:
    grade = "D"
else:
    grade = "F"

print(f"Score: {score}, Grade: {grade}")

hour = 14

if hour < 12:
    greeting = "Good morning!"
elif hour < 17:
    greeting = "Good afternoon!"
elif hour < 21:
    greeting = "Good evening!"
else:
    greeting = "Good night!"

print(greeting)

Multiple if vs. if-elif

An important distinction: multiple if statements each check independently, while if-elif chains stop at the first match.

# Multiple separate if statements - ALL conditions are checked
number = 15

if number > 5:
    print("Greater than 5")    # Runs
if number > 10:
    print("Greater than 10")   # Also runs
if number > 20:
    print("Greater than 20")   # Does not run

# if-elif chain - stops at first True condition
number = 15

if number > 5:
    print("Greater than 5")    # Runs - and Python stops here
elif number > 10:
    print("Greater than 10")   # Skipped even though it's True!
elif number > 20:
    print("Greater than 20")   # Skipped

Use if-elif when the conditions are mutually exclusive (only one should run). Use multiple if statements when multiple conditions could be true simultaneously.

Common Patterns

user_input = input("Enter your age: ")

if user_input.isdigit():
    age = int(user_input)
    print(f"You are {age} years old.")
else:
    print("That's not a valid age!")

a = 15
b = 22

if a > b:
    larger = a
else:
    larger = b

print(f"The larger value is {larger}")

choice = "2"

if choice == "1":
    print("Starting new game...")
elif choice == "2":
    print("Loading saved game...")
elif choice == "3":
    print("Opening settings...")
else:
    print("Invalid choice. Please try again.")